In the Name of Allah, Most Gracious, Most Merciful.

All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.

Dear questioner, first of all, we’d like to say that we are impressed by your question, which emanates from a thoughtful heart. May Allah Almighty help us all adhere to the principles of this true religion, Islam, and enable us to be among the dwellers of Paradise in the Hereafter.

As for your question, if it is proved that the building collapsed because of structural defects, the owner has to bear the price of the two flats. In other words, the buyer does not have to pay the price of the second flat, and the seller has to return the price of the first one. Then he may refer to the contractor or engineer who has cheated him.

But if the building collapsed due to an earthquake, the seller has no hand in it at all. Then the buyer has to pay the price of the two flats so long as he had taken possession of them. But if he had not taken possession of them yet, then they are still under the seller's guarantee, according to the Shafi`i and Maliki schools of fiqh. They agree that real estate remains under the seller's guarantee until the buyer takes possession of it.

Responding to the question, Sheikh Hani ibn `Abdullah Al-Jubair, Judge at the High Court in Jeddah, Saudi Arabia, states the following:

If the construction has collapsed because of some structural defects that the seller did not know about or he knew but did not tell the buyer about, then the buyer does not have to pay the price of the second flat. Moreover, he is entitled to receive back the price of the first one from the seller, because the sold object was defective, and the buyer has right to cancel the deal. Then the seller may refer to the contractor or the engineer who has cheated him.

If, on the other hand, the building has collapsed due to earthquakes, the buyer has to pay the price of the second flat if he had taken possession of it, whether by residing therein or just writing an ownership contract, according to the prevailing norms of registration.

If the buyer had by no means taken possession of the flat, he does not have to pay the price to the seller, as most scholars agree, in contrast to what imam Abu Hanifah and imam Ahmad maintain. They both see that once a real estate is sold, it becomes under the guarantee of the buyer, even if he has not taken possession of it yet.